The name of Inés del Río Prada occupies a particularly grim place in Spain's recent history. Jailed in 1989 for her part in a bombing and murder campaign by the Basque separatist group Eta, she was a member of a unit that killed 24 people, including 12 in a car bombing in the centre of Madrid.

Her release earlier this week at the behest of the European court of human rights (ECHR), prompted a wave of outrage which culminated on Sunday with a march that saw tens of thousands of supporters of the Association of the Victims of Terrorism (AVT) take to the streets of Madrid to protest.

The Spanish courts had little choice but to let Del Río go free. She had been sentenced to 3,828 years in total, but because of a peculiarity of Spanish law dating back to 1973, she could serve a maximum of only 30.

Given time off for good behaviour, Del Río should have been released in 2008. But in 2006, when the Spanish courts realised that she and other Eta prisoners would soon be set free, they introduced the Parot doctrine, which removed the years off for good behaviour.

As with so much of public life in Spain, the issue has become hugely politicised. Calm is going to be hard to restore. It is entirely understandable that the victims of terrorism and their loved ones should want to see Del Río and her cohorts to stay in prison forever. But that is why politicians need to keep a cool head, and why victims do not write the penal code. The AVT is angry because they believe the government, led by Mariano Rajoy of the right-wing People's party (PP), has let them down. The government is angry because the decision to release Del Río was forced on it by the ECHR.

The hard right is angry because it feels it has been made to look weak in the fight against terrorism, for which it blames the previous socialist government of José Luis Rodríguez Zapatero. Pedro J Ramírez, the firebrand editor of El Mundo described Rajoy's leadership as representing the "third term of Zapatero".

Spain has for a long time benefited from European assistance in its fight against terrorism, wrote José Ignacio Torreblanco in Sunday's El País, but now that a European court has taken a decision that many do not like, it cannot suddenly reject European values.

"It is the responsibility of everyone, starting with the government … to defend the freedoms and liberties represented by Europe and embodied by the [ECHR]," he wrote.

Rajoy's government has found itself in a tight spot. When it was in opposition, the PP stood side by side with the AVT, marching alongside them, appearing at press conferences, throwing its support behind the victims of Eta – and guaranteeing their votes. Now the PP is in power, some of the victims have turned against it, denouncing its representatives at Sunday's march as "traitors".

The decision to release Del Río was in fact made by a lower chamber of the Strasbourg court, against which Spain appealed. It was that appeal that the ECHR rejected last Monday. The ruling made matters worse for the Spanish government: the court's rejection of the Parot doctrine does not just apply to Del Río's case, but to all the pending appeals of long-term prisoners who have been denied the right to release for good behaviour.

This means Spain is going to have to set free scores more convicted Eta terrorists, as well as rapists and murderers.

Many observers believe Eta no longer represents a major threat to the unity of Spain, having declared a ceasefire just over two years ago.

However unpalatable, the release of prisoners could yet play a part in the ongoing peace process, helping to move Eta towards a permanent laying down of arms, and allow the Basque Country to move on from its bloody past. But that will require yet more cool heads.